Welan or S-130 is a carbohydrate polymer secreted into the culture medium by a species of gram-negative bacteria, named Alcaligenes (See U.S. Pat. No. 4,342,866). It is a member of a polysaccharide family having the subunit backbone of the general structure I: ##STR2## wherein Glc is glucose, GlcA is glucuronic acid, Rha is rhamnose, Man is mannose, X may be Rha or Man, and wherein the reducing end of the polymer is toward the residue X. This structure (I) is sometimes referred to herein as the "backbone". Various side chains may be attached to the backbone.
Typically members of this polysaccharide family have the general repeating structure II: ##STR3## wherein Glc is glucose; GIcA is glucuronic acid; Rha is rhamnose; Man is mannose; X may be Rha or Man; Z may be .alpha.-L-Rha-(1.fwdarw.6)-.alpha.-L-Rha, .alpha.-L-Man, .alpha.-L-Rha, or Glc; W may be .beta.-D-GIc-(1.fwdarw.6)-.alpha.-D-GIc or .alpha.-L-Rha, subscripts v an y may be either 0, 0.5, or 1, and wherein the reducing end of the polymer is toward the X residue of the backbone. As used herein, the expression "backbone" refers to that portion of structure I which excludes chains W and Z, i.e., when v and y are equal to 0.
Some members of this family of polysaccharides are acetylated at various positions. However, the polysaccharides may be subjected to chemical deacylation in a conventional manner to remove the acyl groups. For example, gellan is a carbohydrate polymer secreted into culture medium by Pseudomonas elodea (ATCC 31461). Gellan has the same carbohydrate backbone as welan (i.e., X=Rha), but lacks the side chain sugar (i.e. v=O and y=O) and the glucose residue 1 is fully substituted with glycerate. The gellan subunit structure is also acylated at unknown positions. Chemical deacylation of gellan produces a polymer which forms a clear gel in the presence of cations. This processed form of gellan is available commercially under the name Gelrite.TM. and is used as an agar substitute for culturing microorganisms and plants.
The gellan family of microbial polysaccharides includes at least seven different polymers which have very similar or identical backbone structures (shown in Table 5 hereinafter in abbreviated form). The known members of this family include: gellan (also called polysaccharide S-60), welan (S-130), S-88, rhamsan (S-194), S-657, S-198, and NW11. All have either gel forming properties or form highly viscous aqueous solutions which make them candidates for commercial applications. However, it is difficult and time consuming to evaluate newly isolated polysaccharides and determine whether or not the new polymer is a member of the gellan family of polysaccharides.
Welan has the following repeat structure III (Jansen, P. -E., Lindberg, B., Widmalm, G. and Sandford, P. A., 1985; Carbohydrate Research 139: 217-223): ##STR4## wherein Glc is glucose; CIcA is glucuronic acid; Rha is rhamnose; and Man is mannose.
As shown above, a side chain composed of either a single mannose or rhamnose sugar is linked to the tetra-saccharide backbone subunit at the glucose residue "2". Although this polymer is negatively charged, due to glucuronic acid residues in the polymer backbone, the polymer is compatible with portland cement. Welan has been proposed as a cement additive because of its strong suspending properties and its ability to reduce entrained air in the cement and to control water loss. See U.S. Pat. Nos. 4,963,668 and 5,004,506. However, for several applications, the native polymer imparts an undesirable high viscosity to the cement.
Methods for chemical cleavage of welan to reduce its viscosity are disclosed in U.S. Pat. Nos. 4,963,668 and 5,004,506. However, the chemical cleavage effected by this method leads to random degradation of the polymer chain and its sugar moieties resulting in a non-uniform chemical composition. This process uses a non-specific oxidation reaction using Fenton's reagent. In order to reduce the amounts of chemicals needed (especially hydrogen peroxide), the patent suggests that the broth be treated first with protease. In addition, the fermentation broth must be heated to about 60.degree. C. (from the fermentation temperature of about 30.degree. C.). Ferrous sulfate (0.05%) and EDTA (a chelation agent, 0.1%) are added first as essential catalysts. Then hydrogen peroxide is added over a 1 to 3 hour period to a final concentration of 0.1.5 to 0.25%. When the broth viscosity is reduced to about 80 to 100 cp, the broth is cooled to about 27.degree. C. and is neutralized with KOH. Welan is recovered by precipitation with isopropyl alcohol.
Viscous solutions of welan are stable in high concentrations of salt and at high temperatures. Because of this stability, welan could be a candidate for oil field applications, since high viscosity fluids are used as suspending agents for hydraulic fracture during well completion. See Baird, J. K., P. A. Sandford and I. W. Cottrell, 1983, Bio/technology:778-783). However, after well completion, it is necessary to reduce the viscosity of the fluids to allow stimulated flow of oil or gas. The prior art processes described above make it difficult and/or expensive to accomplish this with welan.
The use of xanthan gum for hydraulic fracture fluids is known. Specific enzymes are available to reduce the viscosity of xanthan gum, and it has been proposed that these enzymes be used to reduce the viscosity of Xanthan gum-containing fracture fluids (Cadmus, M. C. and M. E. Slodki, 1985; Developments in Industrial Microbiology 26:281-289).